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49 lines
1000 B
C
49 lines
1000 B
C
#if defined(__x86_64__) && defined(__OPTIMIZE__)
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/* WARNING: this only works if compiled with -fomit-frame-pointer */
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void umult64() {
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asm volatile(
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"xchgq %rdx,%rsi\n"
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"movq %rdi,%rax\n"
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"mulq %rdx\n"
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"jc 1f\n" /* overflow */
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"movq %rax,(%rsi)\n"
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"xorq %rax,%rax\n"
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"inc %rax\n"
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"ret\n"
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"1:\n"
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"xorq %rax,%rax\n"
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/* the closing ret is renerated by gcc */
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);
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}
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#else
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#include "safemult.h"
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/* return 1 for overflow, 0 for ok */
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int umult64(uint64 a,uint64 b,uint64* c) {
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uint32 ahi=a>>32;
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uint32 alo=(a&0xffffffff);
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uint32 bhi=b>>32;
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uint32 blo=(b&0xffffffff);
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// a=ahi*x+alo, b=bhi*x+blo
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// a*b = (ahi*x+alo) * (bhi*x+blo)
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// = ahi*x*bhi*x + ahi*x*blo + alo*bhi*x + alo*blo
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// -> overflow if ahi*bhi != zero */
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if (ahi && bhi) return 0;
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a=(uint64)(ahi)*blo+(uint64)(alo)*bhi;
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if (a>0xffffffff) return 0;
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{
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uint64 x=(uint64)(alo)*blo;
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if (x+(a<<32) < x) return 0;
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*c=x+(a<<32);
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}
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return 1;
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}
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#endif
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